Implantable cardiac stimulator with electrode-tissue interface characterization

ABSTRACT

A cardiac stimulator capable of measuring pacing impedance includes a tank capacitor for delivering charge to the heart via device leads, a shunt resistor, and high-impedance buffers for measuring pacing current through the shunt resistor. Soon after the leading edge of the stimulation pulse, the voltage across the shunt resistor, as sampled by a high-impedance buffer, indicates lead and cardiac tissue resistance. Just prior to opening the pacing switch to terminate the stimulation pulse, the voltage across the shunt resistor is sampled by a high-impedance buffer and held once again to allow the capacitance of the lead/heart tissue to be calculated. In alternative embodiments, a high-impedance buffer measures the voltage between the tank capacitor and ground immediately following the stimulation pulse to allow estimation of the lead/heart tissue capacitance. In one alternative embodiment, a look-up table is created in main memory and searched to find the closest lead/heart tissue capacitance estimate to any arbitrary degree of accuracy. In another alternative embodiment, the lead/heart tissue capacitance is estimated by successive approximation to any arbitrary degree of accuracy. When the lead/heart tissue capacitance and lead resistance have been determined, a plurality of parameters of importance for analyzing and optimizing a cardiac stimulation system may be calculated, such as the instantaneous current, the average current, the charge, and the energy delivered to the cardiac tissue.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to implantable cardiacpacing systems and particularly to an improved technique forelectrode-tissue interface characterization. More particularly, thepresent invention relates to an apparatus and method for measuring theresistive and capacitive components of the impedance of pacemaker ordefibrillator leads.

[0003] 2. Background of the Invention

[0004] In the normal human heart, illustrated in FIG. 1, the sinus (orsinoatrial (SA)) node generally located near the junction of thesuperior vena cava and the right atrium constitutes the primary naturalpacemaker by which rhythmic electrical excitation is developed. Thecardiac impulse arising from the sinus node is transmitted to the twoatrial chambers (or atria) at the right and left sides of the heart. Inresponse to excitation from the SA node, the atria contract, pumpingblood from those chambers into the respective ventricular chambers (orventricles). The impulse is transmitted to the ventricles through theatrioventricular (AV) node, and via a conduction system comprising thebundle of His, or common bundle, the right and left bundle branches, andthe Purkinje fibers. The transmitted impulse causes the ventricles tocontract, the right ventricle pumping unoxygenated blood through thepulmonary artery to the lungs, and the left ventricle pumping oxygenated(arterial) blood through the aorta and the lesser arteries to the body.The right atrium receives the unoxygenated (venous) blood. The bloodoxygenated by the lungs is carried via the pulmonary veins to the leftatrium.

[0005] This action is repeated in a rhythmic cardiac cycle in which theatrial and ventricular chambers alternately contract and pump, thenrelax and fill. Four one-way valves, between the atrial and ventricularchambers in the right and left sides of the heart (the tricuspid valveand the mitral valve, respectively), and at the exits of the right andleft ventricles (the pulmonic and aortic valves, respectively, notshown) prevent backflow of the blood as it moves through the heart andthe circulatory system.

[0006] The sinus node is spontaneously rhythmic, and the cardiac rhythmit generates is termed normal sinus rhythm (“NSR”) or simply sinusrhythm. This capacity to produce spontaneous cardiac impulses is calledrhythmicity, or automaticity. Some other cardiac tissues possessrhythmicity and hence constitute secondary natural pacemakers, but thesinus node is the primary natural pacemaker because it spontaneouslygenerates electrical pulses at a faster rate. The secondary pacemakerstend to be inhibited by the more rapid rate at which impulses aregenerated by the sinus node.

[0007] Disruption of the natural pacemaking and propagation system as aresult of aging or disease is commonly treated by artificial cardiacpacing, by which rhythmic electrical discharges are applied to the heartat a desired rate from an artificial pacemaker. An artificial pacemaker(or “pacer”) is a medical device which delivers electrical pulses to anelectrode that is implanted adjacent to or in the patient's heart inorder to stimulate the heart so that it will contract and beat at adesired rate. If the body's natural pacemaker performs correctly, bloodis oxygenated in the lungs and efficiently pumped by the heart to thebody's oxygen-demanding tissues. However, when the body's naturalpacemaker malfunctions, an implantable pacemaker often is required toproperly stimulate the heart. An in-depth explanation of certain cardiacphysiology and pacemaker theory of operation is provided in U.S. Pat.No. 4,830,006.

[0008] Pacers today are typically designed to operate using one of threedifferent response methodologies, namely, asynchronous (fixed rate),inhibited (stimulus generated in the absence of a specified cardiacactivity), or triggered (stimulus delivered in response to a specifiedhemodynamic parameter). Broadly speaking, the inhibited and triggeredpacemakers may be grouped as “demand” type pacemakers, in which a pacingpulse is only generated when demanded by the heart. To determine whatpacing rate is required by the pacemaker, demand pacemakers may sensevarious conditions such as heart rate, physical exertion, temperature,and the like. Moreover, pacemaker implementations range from the simplefixed rate, single chamber device that provides pacing with no sensingfunction, to highly complex models that provide fully automatic dualchamber pacing and sensing functions. The latter type of pacemaker isthe latest in a progression toward physiologic pacing, that is, the modeof artificial pacing that most closely simulates natural pacing.

[0009] Referring now to FIG. 2, a conventional implantable medicaldevice 200 is shown implanted and coupled to a patient's heart 250 byleads 205 and 210. The implantable medical device 200 may include apacemaker or defibrillator or any medical device that performs pacing ordefibrillating functions. The implanted medical device 200 (or simply“pacer”) also includes a housing or “can” 215 which houses a battery andpacing or defibrillating circuitry (not shown). In the dual chamberpacing arrangement shown, leads 205 and 210 are positioned in the rightventricle and right atrium, respectively. Each lead 205 and 210 includesat least one stimulating electrode for delivery of electrical impulsesto excitable myocardial tissue in the appropriate chamber(s) in theright side of the patient's heart. As shown in FIG. 2, each lead 205 and210 includes two electrodes. More specifically, lead 210 includes ringelectrode 230 and tip electrode 235, and lead 205 includes ringelectrode 220 and tip electrode 225. Two, three, and four terminaldevices all have been suggested as possible electrode configurations.

[0010] A lead configuration with two electrodes is known as a “bipolarlead.” Such a configuration typically consists of a pair of wiresarranged coaxially and individually insulated. Each of the wires mayconsist of multiple wire strands wrapped together for redundancy. Acircuit consisting of the pacemaker 200 and the heart muscle can beformed by connecting the lead electrodes to different portions of theheart muscle. In a bipolar configuration, electric current impulsesgenerally flow from the ring electrode through the heart muscle to thetip electrode, although current may travel from the tip electrode to thering electrode in alternative configurations. A lead with one electrodeis known as a “unipolar lead.” In a unipolar configuration, thepacemaker can 215 functions as an electrode. Current flows from theunipolar lead through the heart tissue, returning to the pacer via thecan 215.

[0011] In general, a pacing pulse current is formed by the flow ofcharge carriers in the circuit formed by the lead and tissue. Becausethe electrode is typically composed of a solid conductive material,while the myocardial tissue consists of liquid electrolyte, theelectrode forms an electrode/electrolyte interface through which thecharge carriers pass. Accordingly, electron conductivity accounts forcharge transfer in the lead circuit and in the solid phase of theelectrode interface, while ion conductivity is the primary mechanismresponsible for charge flow through the electrolyte interface andtissues.

[0012] At the interface layer, pacing pulse charge flows from the solidphase of the electrode interface to the electrolyte phase until theelectrochemical potential of the electrode interface balances theelectrochemical potential of the electrolyte interface. During such aprocess, an electric charge layer, known as the Helmholtz layer, formsaround the surface of the electrode. While the exact nature of theHelmholtz layer is very complex, it can be generally modeled as anelectric circuit using voltage sources, diodes, and/or devices thatcontribute impedance (which is the ability to impede electric current)to the lead-tissue circuit. Electrical impedance may be generallycharacterized by the combination of a resistive component, such as aresistor, with a reactive component, such as a capacitor or inductor.One Helmholtz layer model includes a polarization potential (known asthe “Helmholtz voltage”) in series with the parallel combination of aresistor (known as the “Warburg resistor”) and a capacitor (known as the“Helmholtz capacitor”). A second Helmholtz layer model has beensuggested which consists of an impedance circuit shunted by two zenerdiodes. The second configuration accounts for the electrical behavior ofheart tissue when the interface voltage exceeds several hundredmillivolts. A simple yet accurate model of the Helmholtz layer consistsof the Warburg resistance in series with a voltage-dependent Helmholtzcapacitance, eliminating the need to model the polarization potential.

[0013]FIG. 3A illustrates a model of a conventional cardiac stimulatorcircuit consisting of a pacer 200, heart tissue 250, and bipolar pacerlead 205 terminated by tip electrode 225 and ring electrode 220. Ringelectrode 220 and tip electrode 225 couple the pacer 200 to differentportions of the heart tissue 250. Alternatively, a model as in FIG. 3Busing a unipolar lead 305 would include a single electrode 320 coupledto the heart tissue 250 with the pacer can 215 coupled to the chesttissue, labeled as ground. In the unipolar configuration of FIG. 3B, thepacer 200 sends electric current from the pacer can 215 to a singleelectrode 320 through the chest and heart tissue 250. Accordingly, theimpedance introduced by the combination of chest tissue (FIG. 3B only),bipolar lead 205 or unipolar lead 305, and heart tissue 250 may becollectively modeled by resistor R3 (the Warburg resistor) in serieswith capacitor C3 (the Helmholtz capacitor).

[0014] Such models as shown in FIGS. 3A and 3B are important fordelivering “pacing impedance” estimates, which help to indicate thecondition of the pacer leads as well as to estimate electric charge,current, and energy delivered to the heart tissue. Particularly,deviations that occur over time in the pacing impedance serve toindicate the conditions related to the pacing or defibrillation leadsystem. Such conditions include electrode micro-dislocation, leadimpedance changes, evaluation of electrode suitability for detectingevoked potentials, and methods for detecting changes in the excitabletissue as a function of catecholamine concentration, metabolic changes,and ischemia. In addition, the charge, current, energy, and impedancemeasurements allow physicians to estimate the longevity of the implanteddevice. Accordingly, pacing impedance estimates aid physicians inmaintaining and optimizing pacemaker operation throughout the life ofthe device.

[0015] Although a purely resistive lead impedance estimate may provide ameans for a rough estimate of pacer and battery condition, such anestimate may deviate significantly from the true impedance in somesituations, since the physical and electrochemical properties that leadto the Helmholtz layer change with variations in the electric fieldintensity which develops at the electrode-electrolyte interface. Forexample, corrosion, electrocatalysis of glucose and amino acids, andhydrogen ion potentiodynamics drastically alter the modeled capacitance,resistance, and polarization of the interface, as do electrode currentdensity and electric field strength. Further, the Helmholtz capacitancevaries according to a parameter known as the “microsurface area” of theelectrode. The microsurface area of the electrode is the total surfacearea of the electrode material, including microscopic details such asporosity and other microscopic details. Typically, the Helmholtzcapacitance equals about 100 microfarads (μF) per square centimeter ofmicrosurface area. In addition, the resistance, capacitance, andpolarization voltage of the Helmholtz layer can vary according to theduration and amplitude of the pacing pulse, although these propertiesare approximately constant for pulse widths of less than 0.5milliseconds (ms) and pulse amplitudes of less than 0.5 volts (V).

[0016] Methods for measuring the resistive component of pacing impedancehave been available for some time as part of the information thatimplantable pacemakers and defibrillators can collect and telemeter.However, such estimates have neglected the reactive impedance component,as modeled by the Helmholtz capacitance, resulting in an incompletecharacterization of the pacing impedance. Such omissions produceundesirable impedance estimation errors which may propagate intosubsequent calculations of charge, current, and energy delivered to theheart tissue as well as other conditions closely related to the pacingimpedance. Impedance-based methods for monitoring the leads andelectrodes of implantable cardiac stimulators have been described in anumber of patents, including U.S. Pat. Nos. 4,899,750, 5,201,865, and5,534,018 which disclose devices for estimating the resistive leadimpedance component.

[0017] While measurement of the Helmholtz capacitance has been suggestedusing alternating current (AC) circuits, such circuits are not practicalfor use with cardiac stimulation devices, which typically use directcurrent (DC) pulses for cardiac stimulation. Accordingly, devices usingAC methods must operate exclusively of normal pacemaker/defibrillatoroperation. Therefore, no practical device or method for estimating boththe resistive and reactive components of pacer lead impedance has beendevised within a cardiac stimulator, and present-day cardiac stimulatorsmust tolerate the inaccuracies introduced by purely resistive impedanceestimates, as described above.

[0018] For the foregoing reasons, a practical apparatus for measuringboth the resistive and capacitive components of the lead impedance,including the Helmholtz layer, would greatly improve the implementationof implanted stimulation devices. Such an apparatus, if devised, shouldbe adapted to measure lead impedance during normal operation of theimplanted device without affecting the functionality of the pacing ordefibrillating circuit. The resulting device would significantly improvethe accuracy of cardiac impedance estimates, resulting in superioroptimization and maintenance of implanted devices. Unfortunately, todate, no such device is known that provides these features.

SUMMARY OF THE INVENTION

[0019] Accordingly, there is provided herein a cardiac stimulatorincluding a pulse generator for delivering current to the heart tissue,an impedance measurement circuit coupled to the pulse generator, and aprocessor for performing control and calculation functions. Uponreceiving control signals from the processor, the pulse generatortransmits electric current (known as a pacing pulse) from a chargedcapacitor into the heart tissue. At the same time, the processor assertscontrol pulses to the impedance circuit, causing the impedance circuitto sample voltages from the pulse generator. The impedance circuitrecords the voltage measurements through sample-and-hold units,transmitting the voltages as signals to the processor. Using thesevoltage measurements, the processor calculates the impedance of thelead/tissue circuit.

[0020] The pulse generator includes a tank capacitor for deliveringcharge to the heart via device leads and a pacing voltage source forcharging the tank capacitor through an electronically-controlled chargeswitch. Just prior to the time that the pacing pulse is to be deliveredto the heart tissue, the charge switch is opened. A pacing switch isthen closed to allow charge from the tank capacitor to flow through aDC-blocking capacitor into the lead and subsequently the heart. Opposingthe flow of this current are the resistance of the pacing switch, theresistive components of the lead and load impedance (i.e., the leadresistance and ionic resistance), the Helmholtz capacitance, and acurrent-measurement-shunt resistor.

[0021] Soon after the leading edge of the pacing pulse, or at timet=(0⁺), the voltage across the current-measurement-shunt resistor issampled through a high-impedance buffer and held. Since the DC-blockingand Helmholtz capacitances have not charged appreciably at t=(0⁺), theybehave as short-circuits. The pacing circuit is therefore purelyresistive, and the lead and ionic resistance may be calculated by themethod of circuit analysis.

[0022] Just prior to opening the pacing switch to terminate the pacingpulse, or at time t=(T_(PW) ⁻), the voltage across thecurrent-measurement-shunt resistor is sampled by a high-impedance bufferand held once again to allow the Helmholtz capacitance to be calculated.After the pacing pulse is delivered and before the tank capacitor isrecharged, the end voltage of the tank capacitor is sampled through ahigh-impedance buffer and held. Concurrently with the sampling of thetank capacitor end voltage, the DC-blocking capacitor discharges intothe human body by an active discharge switch and a passive-dischargeresistor. In a preferred embodiment, the resistive and capacitivecomponents of the lead impedance may be calculated explicitly using theshunt resistor voltage samples from the high-impedance buffers.

[0023] In other embodiments, the apparatus estimates the Helmholtzcapacitance without knowledge of the voltage across thecurrent-measurement-shunt resistor just prior to the end of the pulse.The voltage across the tank capacitor after the pulse ends, i.e. att=(T_(PW) ⁺), may be expressed using a formula based on pacing voltage,tank capacitance, DC-blocking capacitance, Helmholtz capacitance,current-measurement-shunt resistance, pacing switch resistance,lead/tissue resistance, and pulse width, all of which are known valuesexcept the Helmholtz capacitance and lead/tissue resistance. The tankvoltage formula consists of an exponential term multiplied by a constantterm and added to an additive term. All three terms include theHelmholtz capacitance as a variable. If the tank capacitor voltage ismeasured following the pulse and the lead/tissue resistance iscalculated using circuit analysis as above, then the formula reduces toan equation involving only one unknown variable, the Helmholtzcapacitance.

[0024] In an alternative embodiment, a look-up table is created in mainmemory by using the calculated Warburg resistance combined with knownvalues of the pacing voltage, tank capacitance, DC-blocking capacitance,current-measurement-shunt resistance, pacing switch resistance, andpulse width in the formula along with a series of empirical estimatesfor the value of the Helmholtz capacitance. The formula produces adistinct tank capacitor voltage calculation for each Helmholtzcapacitance estimate. The Helmholtz capacitance estimates along with thecalculated tank capacitor voltages are stored into main memory as alook-up table, and the actual, measured tank capacitor voltage iscompared with the set of calculated tank capacitor voltages. Searchingthrough the look-up table, the apparatus chooses the Helmholtzcapacitance estimate as the empirical estimate which produced acalculated tank capacitor voltage that most closely resembles themeasured tank capacitor voltage.

[0025] In another embodiment, a single empirical estimate for theHelmholtz capacitance is substituted into the one part of the formula,either into the exponential term or into the additive and constantterms. The remaining term(s) may be reduced algebraically to solve forthe unknown Helmholtz capacitance value. If the resulting calculation ofthe Helmholtz capacitance value does not agree with the originallysubstituted empirical estimate, then an updated empirical estimate issubstituted into the first term(s), and a new Helmholtz capacitance iscalculated using the remaining term(s). If the resulting calculation ofthe Helmholtz capacitance value lies within an acceptable range of theoriginally substituted empirical estimate, then the measured Helmholtzcapacity is determined as the final empirical estimate. Such anapproximation is simple to compute using conventional circuitry and canconform to any arbitrary level of accuracy by iterating through theequation with progressively better estimates for the Helmholtzcapacitance.

[0026] When the Helmholtz capacitance and Warburg resistance have beendetermined, a plurality of parameters of importance for analyzing andoptimizing a pacing system may be calculated, including the currentdelivered to the cardiac tissue at any instantaneous point in time, theaverage current delivered to the cardiac tissue over the duration of thepulse, the total charge and the total energy delivered to the cardiactissue and to the leads, and the Helmholtz potential after pacingpolarization.

[0027] Thus, the present invention comprises a combination of featuresand advantages that enable it to substantially advance the art byproviding an apparatus for gauging both the resistive and capacitivecomponents of the Helmholtz layer. These and various othercharacteristics and advantages of the present invention will be readilyapparent to those skilled in the art upon reading the following detaileddescription of the preferred embodiments of the invention and byreferring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] A better understanding of the present invention can be obtainedwhen the following detailed description of the preferred embodiment isconsidered in conjunction with the following drawings, in which:

[0029]FIG. 1 illustrates the human heart;

[0030]FIG. 2 shows the typical connections between a conventionalpacer-defibrillator and the human heart;

[0031]FIG. 3A is a known model of the Helmholtz circuit for a bipolarlead configuration;

[0032]FIG. 3B is a known model of the Helmholtz circuit for a unipolarlead configuration;

[0033]FIG. 4 is an exemplary block diagram of a cardiac stimulator madein accordance with the present invention;

[0034]FIG. 5 is a block diagram of the impedance circuit and pulsegenerator circuit of the cardiac stimulator shown in FIG. 4;

[0035]FIG. 6 is a timing diagram showing the control signals asserted bythe processor of the cardiac stimulator shown in FIG. 4;

[0036]FIG. 7 is a graph of the voltage across the tank capacitor of FIG.5 versus the Helmholtz voltage created in the heart tissue duringcardiac stimulation; and

[0037]FIG. 8 is a flowchart describing an alternative embodiment forestimating the Helmholtz voltage using the apparatus of FIG. 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0038] An exemplary cardiac stimulator 400 made in accordance with thepresent invention is illustrated in the block diagram of FIG. 4. Thecardiac stimulator 400 may be a pacemaker, a defibrillator, or any orimplantable cardiac stimulator. The cardiac stimulator 400 generallyincludes atrial and ventricular sense circuits 462 and 464, a processor470, main memory 475, an impedance circuit 466, and a pulse generator468, all housed in an enclosure, or “can” 401. The exemplary embodimentof FIG. 4 shows cardiac stimulator 400 with four leaded electrodes,namely atrial tip and ring electrodes 410 and 420, respectively, andventricular ring and tip electrodes 440 and 450, respectively. Can 401may function as an additional electrode in accordance with knowntechniques. The invention, however, may be practiced using any number ofelectrodes implanted in any chamber of the heart and in anyconfiguration.

[0039] Referring still to FIG. 4, electrodes 410 and 420 couple to theatrial sense circuit 462 via capacitors C1 and C2, respectively, whichare preferably 0.15 microfarad (μF) capacitors. Similarly, electrodes440 and 450 couple to the ventricular sense circuit 464 via capacitorsC3 and C4, respectively, which are also preferably 0.15 μF capacitors.The atrial sense circuit 462 processes signals received from the atrialchamber of the heart via the atrial electrodes 410 and 420, while theventricular sense circuit 464 processes signals received from theventricular chamber via the ventricular electrodes 440 and 450. Theatrial and ventricular sense circuits 462 and 464 generally include alow power, highly sensitive amplifier, a band pass filter, and athreshold detector (not shown). The atrial 462 and ventricular 464circuits further include internal pulldown switches SW_(A) and SW_(V),respectively, the states of which are controlled by the processor 470.The amplifier amplifies the electrical signal from the associatedelectrodes, and the band pass filter attenuates signals whosefrequencies are outside the range of frequencies known to correspond tocardiac signals. The threshold detector compares the amplified andfiltered signal to a reference signal to determine when a cardiac event(also referred to as a “sense event”) has occurred. If the magnitude ofthe amplified and filtered cardiac signal exceeds the reference signal,the processor 470 determines that a sense event has occurred. Theprocessor 470 may then pace the heart based either on detecting or notdetecting sense events via pulse generator 468 and electrodes 401, 410,420, 440, and 450. For example, the processor 470 may initiate aventricular pacing pulse if an atrial sense event has not been detectedwithin a predetermined period of time following a previous atrial senseevent.

[0040] Cardiac stimulator 400 further includes lead switches SW1 and SW2as well as can switch SW3 for configuring unipolar and bipolar sensingmodes and also unipolar and bipolar pacing modes, as described below.Switches SW1, SW2, and SW3 are preferably processor-controlled,single-pole single-throw (SPST) switches. When closed by the processor470, the atrial lead switch SW1 couples the atrial ring electrode 420 toground. Similarly, the ventricular lead switch SW2, when closed by theprocessor 470, couples the ventricular ring electrode 450 to ground. Canswitch SW3, when closed by the processor 470, couples the can 401 toground.

[0041] For atrial sensing using bipolar leads, atrial lead switch SW1,atrial internal pulldown switch SW_(A), and can switch SW3 are allpreferably open. In this configuration, the atrial sense circuit 462receives a differential sense signal from tip 410 and ring 420electrodes, respectively. For atrial sensing using a unipolar leadconfiguration, atrial lead switch SW1 remains open, but atrial internalpulldown switch SW_(A) and atrial can switch SW3 are preferably closed.

[0042] Ventricular sensing operates in substantially the same manner.For ventricular sensing using bipolar leads, ventricular lead switchSW2, ventricular internal pulldown switch SW_(V), and can switch SW3 areall preferably open. In this configuration, the ventricular sensecircuit 464 receives a differential sense signal from tip 440 and ring450 electrodes, respectively. For ventricular sensing using a unipolarlead configuration, ventricular lead switch SW2 remains open, butventricular internal pulldown switch SW_(V) and can switch SW3 arepreferably closed.

[0043] The pulse generator 468 produces an appropriate electrical pulseto stimulate the desired chamber of the heart to beat. The processor 470initiates the pulse generator 468 to produce a pacing pulse, and thepulse generator responds by delivering the pacing pulse to the desiredchamber of the heart. The pulse generator 468 preferably includes a ratelimiter to prevent the processor 470 from erroneously pacing the heartat an excessively high rate. The pulse generator 468 preferably couplesto the atrial tip electrode 410 via an atrial pulse line 480 in serieswith a DC-blocking series capacitor C5 and further couples toventricular tip electrode 440 via a ventricular pulse line 485 in serieswith a DC-blocking series capacitor C6. Further, the pulse generator 468couples to ground to provide a circuit return path for pacing pulses.Hence, the pulse generator 468 may send a pacing pulse to the atrial orventricular chamber via atrial pulse line 480 or ventricular pulse line485, respectively.

[0044] In addition to selecting atrial or ventricular sensing, switchesSW1, SW2, and SW3 configure the cardiac stimulator 400 for unipolar orbipolar pacing. For atrial bipolar pacing, atrial lead switch SW1 ispreferably closed (therefore coupled to ground), and can switch SW3 isopen. This bipolar pacing configuration allows a pacing pulse issued tothe atrial chamber via atrial pulse line 480 and atrial tip electrode410 to complete a circuit path to the pulse generator 468 through atrialring electrode 420, which couples to ground. Ventricular bipolar pacingoccurs in substantially the same manner, with ventricular lead switchSW2 closed (therefore coupled to ground) and can switch SW3 open. Apacing pulse issued to the ventricular chamber via ventricular pacingline 485 is then allowed to complete a circuit path to the pulsegenerator 468 through ventricular ring electrode 450, which couples toground.

[0045] For unipolar stimulation, can switch SW3 is closed, and atriallead switch SW1 (for stimulation of the atrial chamber) or ventricularlead switch SW2 (for stimulation of the ventricular chamber) is opened.In this unipolar pacing configuration, a pacing pulse issued to theatrial chamber via atrial pacing line 480 and atrial tip electrode 410is allowed to complete a circuit path to the pulse generator 468 via thecan 410, which is coupled to ground. Similarly, a pacing pulse issued tothe ventricular chamber via ventricular pacing line 485 and ventriculartip electrode 450 is allowed to complete a circuit path to the pulsegenerator 468 via the can 410, which is coupled to ground.

[0046] Main memory 475 couples to the processor 470 and is capable ofstoring program instructions and other data to be retrieved or updatedby the processor 470. Accordingly, cardiac stimulator 400 may beprogrammed through instructions stored in main memory to operate in oneof a number of pacing modes. For example, the cardiac stimulator 400 maybe programmed to sense electrical activity in the atrium, and then topace the ventricle following a predetermined time delay after theoccurrence of an atrial sense event if the ventricle has not contractedon its own. Additionally, the processor 470 may be programmed to storesense data, impedance data, or other information in main memory 475 tobe retrieved at a later date either by the processor 470 or by aphysician.

[0047] Cardiac stimulator 400 uses an impedance circuit 466 to determinethe electrical impedance of the lead and heart tissue circuit, asmodeled by FIGS. 3A and 3B. The impedance circuit 466 generallyprocesses the electrical signal from the pulse generator 468 andprovides one or more output status signals to the processor 470. Theprocessor 470 uses the status signal from the impedance circuit 466 tocompute the impedance of the lead/heart tissue, as described in moredetail below.

[0048]FIG. 5 illustrates the electrical characteristics of theresistance of lead 505 combined with the impedance inherent in heart250. Resistor R_(L) generally represents the combined resistance of thelead 505 and the heart 250, while C_(L) represents the Helmholtzcapacitance described previously. Note that R_(L) and C_(L) do notdepict actual components in the present invention but represent a modelof the lead/heart tissue impedance to be determined. Cardiac stimulator400 calculates lead/tissue resistance R_(L) and Helmholtz capacitanceC_(L) in accordance with the methods described below. Referring still toFIG. 5, a preferred embodiment of a pulse generator 468 is shown coupledto heart 250 via lead 505. Pulse generator 468 comprises a voltagesource V_(i), a charge switch SW1, a pacing switch SW2, tank capacitorC_(T), current-measurement-shunt resistor R_(T), a discharge switch SW3,discharge resistor R_(X), and DC-blocking capacitor C_(B).

[0049] Voltage source V_(i) is any suitable voltage source for chargingtank capacitor C_(T). Voltage source V_(i) typically comprises a batterywhich may or may not be rechargeable and a programmable voltagemultiplier. Voltage source V_(i) couples to charging switch SW1, whichpreferably is a single-pole/single-throw (SPST) switch controlled by aprocessor such as processor 470 in FIG. 4, via a charge control signal525. Tank capacitor C_(T) and shunt resistor R_(T) couple in seriesbetween charging switch SW1 and ground, with C_(T) connected directly toSW1 and R_(T) connected directly to ground. One terminal of pacingswitch SW2 connects between charge switch SW1 and tank capacitor C_(T)while the other terminal of switch SW2 connects to a DC-blockingcapacitor C_(B), discharge switch SW3, and discharge resistor R_(X).Pacing switch SW2 is preferably an SPST switch with an internal switchresistance R_(SW). Processor 470 controls the state of pacing switch SW2via a pace control signal 530. Switch SW3 likewise is aprocessor-controlled, SPST switch, coupling to the processor 470 via adischarge control signal 535. Discharge switch SW3 and dischargeresistor R_(X) further couple in parallel and connect to ground.Discharge resistor R_(X) preferably has a very high resistance comparedwith shunt resistor R_(T), switch resistance R_(SW), and lead/tissueresistance R_(L). A preferred embodiment includes a shunt resistor R_(T)of 22 Ω (ohms), a switch resistance R_(SW) of 10 Ω, a discharge resistorR_(X) of 100 kΩ, and a typical lead/tissue resistance of 500 Ω.

[0050] Lead 505 couples to DC-blocking capacitor C_(B) and terminates toelectrode 520 at the heart 250. While lead 505 preferably compriseseither a bipolar or unipolar lead, it is illustrated in FIG. 5 as aunipolar lead for simplicity. As one of ordinary skill in the art wouldrecognize, the circuits of FIGS. 3A and 3B are substantially the same,since the ground node essentially serves as a lead substitute byproviding a current path from the cardiac stimulator 400 to the heart.Thus, the circuit of FIG. 5 applies equally to both bipolar and unipolarlead configurations.

[0051] Impedance circuit 466 preferably comprises three sample-and-holdunits U1, U2, and U3, as well as a pair of high-impedance buffers U4 andU5. Each buffer U4 and U5 may comprise any buffer circuit configured asa voltage follower with high-impedance inputs. The buffers U4 and U5 inthe present embodiment are shown as unity-gain operational amplifiers(or “op-amps”), with each buffer output coupled directly to theinverting input (−) of the same buffer. Alternatively, the buffers mayconsist of any device that amplifies an input signal. The invertinginputs of buffers U4 and U5 connect to resistors R1 and R2,respectively, which also couple to ground. The noninverting input (+) ofbuffer U4 couples to tank capacitor C_(T), charging switch SW1, andpacing switch SW2. The noninverting input of buffer U5 couples to thejunction between tank capacitor C_(T) and shunt resistor R_(T). Theoutput of buffer U4 drives the input of sample-and-hold unit U1. Theoutput of buffer U5 drives both sample-and-hold units U2 and U3.

[0052] The sample-and-hold units are controlled by the processor viasignals sample1 540 (U1), sample2 545 (U2), and sample3 550 (U3). When asample control signal 540, 545, or 550 is asserted or pulsed, thecorresponding sample-and-hold unit instantaneously samples the voltageappearing on its input terminal and holds that voltage on its outputterminal even after the input signal is changed or removed. As describedbelow, the output signals from sample-and-hold units U1, U2, and U3represent voltages measured in the pulse generator 468. In a preferredembodiment, voltages are sampled at specific times in relation to thepacing pulse. For a pacing pulse with a duration of T_(PW) seconds,sample-and-hold unit U3 will sample the shunt resistor voltage justafter the beginning of the pacing pulse, sample-and-hold unit U2 willsample the shunt resistor voltage just before the end of the pacingpulse, and sample-and-hold unit U1 will sample the tank capacitorvoltage following the pacing pulse. A more detailed explanation of thesevoltages readings is presented below, with respect to FIG. 6. Thehigh-impedance nature of buffers U4 and U5 insures that the pulsegenerator 468 voltages are measured with negligible interference to thepulse generator 468.

[0053] Still referring to FIG. 5, voltage source V_(i) charges tankcapacitor C_(T) to a voltage substantially equivalent to V_(i) when thecharging switch SW1 is closed. When the charging switch SW1 anddischarging switch SW3 are opened and pacing switch SW2 is subsequentlyclosed, the tank capacitor C_(T) and shunt resistor R_(T) areeffectively switched into a resistive-capacitive (or “RC”) chargingcircuit including switch resistance R_(SW), discharge resistor R_(X),DC-blocking capacitor C_(B), lead/tissue resistance R_(L), and Helmholtzcapacitance C_(L). Thus, the charge stored in C_(T) discharges intoR_(T), R_(SW), R_(X), C_(B), R_(L), and C_(L).

[0054]FIG. 6 illustrates a detailed timing diagram of the controlsignals sample1, sample2, sample3, pace, discharge, and charge which areasserted by the processor 470 of FIG. 5 to control the pulse generator468 and impedance circuit 466. In the diagram of FIG. 6, the pacingpulse begins at t=0 and preferably extends for a duration of T_(PW)seconds. Prior to the beginning of the pacing pulse, the charge anddischarge signals are held low, or asserted, causing the charging switchSW1 and discharging switch SW3 to close. Also prior to the beginning ofthe pacing pulse, the pace signal is held high, or deasserted, causingthe pacing switch SW2 to open. Thus, the tank capacitor C_(T) charges toV_(i) volts. In a preferred embodiment, sample1, sample2, and sample3remain low prior to the beginning of the pacing pulse at time t=0,indicating that the previous samples are being held at the outputs ofsample-and-hold units U1, U2, and U3. The tank capacitor C_(T) becomessufficiently charged prior to time t=0, and the processor 470 deassertsthe charge and discharge signals at points 600 and 605, respectively.

[0055] The pacing pulse begins at time t=0 when the processor 470asserts the pace signal (point 610) to a logic low state, allowingcharge from the tank capacitor C_(T) to begin flowing into thelead/tissue circuit. At time t=0⁺, which preferably is less than 10 safter time t=0, the processor 470 pulses sample3 (point 615), causingsample-and-hold unit U3 to record the voltage V_(RT)(0⁺) across theshunt resistor R_(T). The tank capacitor C_(T) continues to dischargeuntil the end of the pacing pulse at time t=T_(PW), which is marked bypoint 630. At time t=T_(PW) ⁻, however, which preferably occursapproximately 10 s or less before time t=T_(PW), the processor 470pulses sample2 (point 620), causing sample-and-hold unit U2 to recordthe voltage V_(RT)(T_(PW) ⁻) across the shunt resistor.

[0056] At time t=T_(PW), the processor 470 halts the pacing pulse bydeasserting the pace signal (point 630) to a logic high state.Subsequently, the electric charge accumulated in the DC-blockingcapacitor C_(B) and the Helmholtz layer (represented by C_(L)) begins todischarge to ground through the discharge resistor R_(X). In alternativeembodiments, the processor pulses sample1 (point 635) at time T_(PW) ⁺,which preferably occurs approximately 10 s or less after time t=T_(PW).Next, the processor 470 asserts the discharge and charge signals atpoints 640 and 645, respectively. The discharge signal allows anyelectric charge remaining in the DC-blocking capacitor C_(B) andHelmholtz layer (C_(L)) to quickly discharge, while the charge signalcauses voltage source V_(i) to charge tank capacitor C_(T) inpreparation for delivering the next pacing pulse.

[0057] Any capacitor behaves as a short-circuit for a short time aftercurrent is applied to that capacitor. Thus, immediately after tankcapacitor C_(T) and shunt resistor R_(T) are switched into the chargingcircuit, or at time t=0⁺, the current in the charging circuit equals thevoltage held by C_(T) divided by the resistance presented by theresistive circuit of R_(X), R_(T), R_(SW), and R_(L). At the same time,processor 470 asserts control signal sample3, causing sample-and-holdunit U3 to sample and hold the voltage drop V_(RT)(0⁺) across shuntresistor R_(T). Because the voltage drop across any resistor isproportional to the current flowing through that resistor, the voltageV_(RT)(0⁺) can be used to determine the current flowing through thecharging circuit. It follows that the lead/tissue resistance R_(L) canbe calculated using equation (1) below: $\begin{matrix}{R_{L} = {- \frac{R_{X}}{\frac{R_{X}}{{R_{T}\left( {\frac{V_{i}}{V_{RT}\left( 0^{+} \right)} + 1} \right)} + R_{S\quad W}} + 1}}} & (1)\end{matrix}$

[0058] When a constant voltage is applied to an RC circuit, the amountof current flowing through that circuit changes over time in awell-documented manner. Thus, as the charge contained in tank capacitorC_(T) is released into the charging circuit from time t=0 to timet=T_(PW), the charging current changes over time. The rate at which thecurrent changes is determined by the resistances R_(T), R_(SW), andR_(L) and capacitances C_(T), C_(B), and C_(L).

[0059] Because the voltage drop across the shunt resistor at any pointin time V_(RT)(t) is directly proportional to the current through R_(T)and because the resistances R_(T), R_(SW), and R_(L) and capacitancesC_(T), C_(B), and C_(L) uniquely determine the charging current at timet=T_(PW) ⁻, the Helmholtz capacitance C_(L) may be calculated usingequation (2) below. Because R_(X) has a very high impedance comparedwith the remaining components in the circuit, little current flowsthrough R_(X). Thus, the presence of R_(X) may be neglected for purposesof analyzing the Helmholtz capacitance C_(L). $\begin{matrix}{C_{L} = {- \frac{C_{T}C_{B}}{\begin{matrix}{{C_{T}C_{B}\frac{R_{T} + R_{S\quad W} + R_{L}}{T_{P\quad W}}{\ln \left( {- \frac{{V_{R\quad T}\left( T_{P\quad W}^{-} \right)}\left\lbrack {R_{T} + R_{S\quad W} + R_{L}} \right\rbrack}{V_{i}R_{T}}} \right)}} +} \\{C_{B} + C_{T}}\end{matrix}}}} & (2)\end{matrix}$

[0060] where ln( ) is the natural logarithm function.

[0061] Following the charging pulse, sample-and-hold units U3 and U2hold voltages V_(RT)(0⁺) and V_(RT)(T_(PW) ⁻), respectively. Using thesemeasured values of V_(RT)(0⁺) and V_(RT)(T_(PW) ⁻) along with knownvalues of C_(T), R_(T), and R_(SW), the processor 470 calculates thelead/tissue resistance R_(L) and the Helmholtz capacitance C_(L) usingequations (1) and (2), above. These calculations provide an accuratecharacterization of the lead/tissue impedance and assist physicians inmonitoring lead integrity, device longevity, and current, charge, andenergy delivered to the heart tissue.

[0062] The pulse generator 468 operates as described previously, and theprocessor 470 asserts sample3 at time t=0⁺to measure the shunt resistorvoltage V_(RT)(0⁺) at the beginning of the pulse period. Shortly aftertime t=T_(PW), or at time t=T_(PW) ⁺, the processor 470 asserts thesample1 control signal to cause and sample-and-hold unit U1 to recordthe voltage of tank capacitor C_(T) via buffer U4 immediately followingthe pulse period. The time t=T_(PW) ⁺ is preferably less than 10 s aftertime t=T_(PW). The tank capacitor voltage at time t=T_(PW) ⁺, orV_(CT)(T_(PW) ⁺), represents the voltage across tank capacitor C_(T)with respect to ground shortly after the pulse period. The measurementof V_(RT)(0⁺) allows the processor 470 to calculate the lead/tissueresistance R_(L) as before, using equation (1). In the alternativeembodiment, however, the processor 470 uses V_(CT)(T_(PW) ⁺) in equation(3), below, to estimate the Helmholtz capacitance C_(L) either bygenerating a lookup table or by successive approximation, as will beexplained below with respect to FIGS. 8A and 8B. Equation (3) governsthe tank capacitor voltage at time t=T_(PW) ⁺: $\begin{matrix}{{V_{CT}\left( T_{P\quad W}^{+} \right)} = {\frac{V_{i}\left( {{C_{T}C_{B}} + {C_{T}C_{L}}} \right)}{{C_{T}C_{B}} + {C_{T}C_{L}} + {C_{B}C_{L}}} + {{V_{i}\left( {1 - \frac{{C_{T}C_{B}} + {C_{T}C_{L}}}{{C_{T}C_{B}} + {C_{T}C_{L}} + {C_{B}C_{L}}}} \right)}^{\frac{{- {({\frac{1}{C_{T}} + \frac{1}{C_{S}} + \frac{1}{C_{L}}})}}T_{P\quad W}^{*}}{R_{T} + R_{S\quad W} + R_{L}}}}}} & (3)\end{matrix}$

[0063] where e is the base of the natural logarithm.

[0064]FIG. 7 illustrates a graph of V_(CT)(T_(PW) ⁺) versus C_(L),according to equation (3). Note that for any point on the graph, anincrease in Helmholtz capacitance C_(L) results in a decrease inV_(CT)(T_(PW) ⁺). For example, point 700 represents C_(L)=10 μF,V_(CT)(T_(PW) ⁺)=4.06. It can be seen that for any C_(L)>10 μF,V_(CT)(T_(PW) ⁺)<4.06. For instance, C_(L)=15 μF and V_(CT)(T_(PW)⁺)=4.02 at point 705. Thus, V_(CT)(T_(PW) ⁺) of equation (3) is said tomonotonically decrease in Helmholtz capacitance C_(L). It follows thatany measured tank capacitor voltage V_(CT)(T_(PW) ⁺) corresponds to aunique Helmholtz capacitance C_(L) which may be calculated using thealternative embodiments presented herein.

[0065] After the processor 470 calculates the lead/tissue resistanceR_(L) using shunt resistor voltage measurement V_(RT)(0⁺) in equation(1), all the variables in equation (3) are known except for theHelmholtz capacitance C_(L). To determine C_(L), note that theright-hand side of equation (3) consists of an additive term A=${A = \frac{V_{i}\left( {{C_{T}C_{B}} + {C_{T}C_{L}}} \right)}{{C_{T}C_{B}} + {C_{T}C_{L}} + {C_{B}C_{L}}}},$

[0066] a constant term K=${K = {V_{i}\left( {1 - \frac{{C_{T}C_{B}} + {C_{T}C_{L}}}{{C_{T}C_{B}} + {C_{T}C_{L}} + {C_{B}C_{L}}}} \right)}},{a\quad n\quad d}$

${a\quad n\quad e\quad x\quad p\quad o\quad n\quad e\quad n\quad t\quad i\quad a\quad l\quad t\quad e\quad r\quad m\quad E} = {^{\frac{{- {({\frac{1}{C_{T}} + \frac{1}{C_{B}} + \frac{1}{C_{L}}})}}T_{P\quad W}^{+}}{R_{T} + R_{S\quad W} + R_{L}}}.}$

[0067] Because the Helmholtz capacitance C_(L) is present in theadditive, constant, and exponential terms in equation (3), there is noexplicit algebraic solution for C_(L). Hence, in one alternativeembodiment, the processor 470 either generates or retrieves from memorya set of candidate estimates for Helmholtz capacitance C_(L). Theprocessor then evaluates the right-hand-side of equation (3) using eachof the candidate estimates, recording the evaluation results into memoryas a lookup table. The processor 470 estimates C_(L) by determiningwhich evaluation of equation (3) most closely matches the voltageV_(CT)(T_(PW) ⁺) at the output of sample-and-hold unit U1. BecauseV_(CT)(T_(PW) ⁺) in equation (3) decreases monotonically in C_(L), thevalue of C_(L) used in equation (3) to compute the V_(CT)(T_(PW) ⁺)which most closely matches the V_(CT)(T_(PW) ⁺) measured from U1 is agood estimate of the actual Helmholtz capacitance, C_(L). Further, theprocessor 470 may be programmed to estimate the Helmholtz capacitance toany arbitrary degree of accuracy in this embodiment by evaluatingequation (3) using numerous candidate values of C_(L) which aresufficiently closely spaced.

[0068] Table I illustrates an exemplary lookup table using thisalternative embodiment. To generate Table I, processor 470 uses knownvalues of V_(i), C_(T), C_(B), R_(T), R_(SW), and T_(PW) which have beenpreviously stored in processor memory. For purposes of this example,these values are V_(i)=5 V, C_(T)=10 μF, C_(B)=10 μF, R_(T)=22 Ω,R_(SW)=17 Ω, and T_(PW) ⁺=1.5 ms. Also, a set of candidate values forC_(L) has been stored into the processor 470. For purposes of thisexample, these values are 1 μF, 2 μF, 3 μF, 4 μF, 5 μF, 6 μF, 7 μF, 8μF, 9 μF, and 10 μF. Assuming also for this example that the processoruses the output of sample-and-hold unit U3 to calculate the lead/tissueresistance R_(L)=500 Ω, the processor evaluates equation (3) using eachof the candidate values of C_(L). Table I illustrates the resultingcalculations of V_(CT)(T_(PW) ⁺) as a function of the candidate C_(L)values. TABLE I Example lookup table calculated from equation (3) andused to estimate C_(L). C_(L) V_(CT)(T_(PW) ⁺) (candidate) (calculated)1 μF 4.5981 V 2 μF 4.3875 V 3 μF 4.2750 V 4 μF 4.2065 V 5 μF 4.1606 V 6μF 4.1279 V 7 μF 4.1033 V 8 μF 4.0843 V 9 μF 4.0690 V 10 μF  4.0565 V

[0069] In this example, the processor 470 measures from sample-and-holdunit U1 the actual tank capacitor voltage after the pulse, orV_(CT)(T_(PW) ⁺), as 4.08 V. Scanning through the lookup table, theprocessor determines that the measured value of V_(CT)(T_(PW) ⁺) mostclosely matches the lookup table value 4.0843 V. Because C_(L)=8 μFcorresponds to V_(CT)(T_(PW) ⁺)=4.0843, the processor determines C_(L)to be 8 μF in this example. Note that the impedance values, voltages,pulse width, and candidate C_(L) values described herein are used onlyfor this example and are not intended to limit the present invention.Furthermore, a lookup table of this embodiment may have any number andrange of candidate C_(L) values and should not be limited to thecandidate C_(L) values presented in the example.

[0070] In another alternative embodiment, the processor 470 calculatesthe lead/tissue resistance R_(L) and measures the tank capacitor voltagefollowing the pacing pulse V_(CT)(T_(PW) ⁺) as before. In thisembodiment, however, the processor uses equation (3) to iterativelyestimate the Helmholtz capacitance C_(L). First, the processor 470substitutes an empirical estimate, preferably greater than the largestpossible Helmholtz capacitance C_(L), into the exponential term ofequation (3). The processor then solves for an approximation of C_(L) inthe additive and constant terms. If the empirical estimate of C_(L)agrees closely with the calculated approximation, then the processoruses the calculated approximation for the Helmholtz impedance.

[0071] The flowchart of FIG. 8 illustrates the steps of successiveapproximation involved in this embodiment if the processor inserts theempirical estimate of C_(L) into the exponential term of equation (3)and solves for an approximation of C_(L) using the additive and constantterms. The flowchart begins at the “start” block. Moving to block 800,the processor 470 computes the value of the exponential term E=e$E = ^{\frac{{- {({\frac{1}{C_{T}} + \frac{1}{C_{B}} + \frac{1}{C_{L}{({e\quad m\quad p\quad i\quad r\quad i\quad c\quad a\quad l})}}})}}T_{P\quad W}^{+}}{R_{T} + R_{S\quad W} + R_{L}}}$

[0072] using an initial empirical estimate of C_(L), orC_(L)(empirical), that is preferably larger than the largest possibleC_(L) value. Using the calculated E, equation (3) may be expressed as inequation (4), below, which permits solving for C_(L) algebraically.$\begin{matrix}{{V_{CT}\left( T_{P\quad W}^{+} \right)} = {\frac{V_{i}\left( {{C_{T}C_{B}} + {C_{T}C_{L}}} \right)}{{C_{T}C_{B}} + {C_{T}C_{L}} + {C_{B}C_{L}}} + {{V_{i}\left( {1 - \frac{{C_{T}C_{B}} + {C_{T}C_{L}}}{{C_{T}C_{B}} + {C_{T}C_{L}} + {C_{B}C_{L}}}} \right)}E}}} & (4)\end{matrix}$

[0073] In block 805, the processor 470 solves equation (4) algebraicallyfor C_(L), resulting in an approximation of the Helmholtz capacitanceC_(L)(approx). The algebraic solution for C_(L) in equation (4) is givenby C_(L)(approx) in equation (5): $\begin{matrix}{{C_{L}\left( {a\quad p\quad p\quad r\quad o\quad x} \right)} = \frac{C_{T}{C_{B}\left( {V_{i} - {V_{CT}\left( T_{P\quad W}^{+} \right)}} \right)}}{{\left( {C_{T} + C_{B}} \right){V_{C\quad T}\left( T_{P\quad W}^{+} \right)}} - {V_{i}C_{T}} - {V_{i}E\quad C_{B}}}} & (5)\end{matrix}$

[0074] In block 810, the processor computes the absolute differencebetween C_(L)(empirical) and C_(L)(approx), or|C_(L)(empirical)−C_(L)(approx)|. If the absolute difference betweenC_(L)(empirical) and C_(L)(approx) is greater than a predetermined limitΔ_(CL), which is preferably Δ_(CL)=1 μF, then the processor 470 moves toblock 815 and adjusts the empirical estimate C_(L)(empirical) so thatthe absolute difference between C_(L)(empirical) and C_(L)(approx) issmaller during a subsequent iteration. Because of the nature of thisprocedure, equation (5) always produces a value of C_(L)(approx) that isbetween C_(L)(empirical) and the true Helmholtz capacitance. Thus,C_(L)(empirical) is preferably adjusted by setting C_(L)(empirical)equal to C_(L)(approx), although other known methods of adjustingC_(L)(empirical) so that C_(L)(empirical) and C_(L)(approx) convergeiteratively may be used as well. When C_(L)(empirical) is adjusted toproduce a new C_(L)(empirical) in step 815, the processor 470 repeatssteps 800, 805, 810, and 815 of the flowchart until C_(L)(approx) iswithin the predetermined limit Δ_(CL) of C_(L)(empirical).

[0075] Next moving to step 820, the processor 470 determines ifC_(L)(empirical) is greater than C_(L)(approx). If C_(L)(empirical) isgreater than C_(L)(approx) in step 820, then the currentC_(L)(empirical) is larger than the true Helmholtz capacitance, and theprocessor moves to step 825. In step 825, C_(L)(empirical) is preferablyadjusted by subtracting Δ_(CL) from C_(L)(empirical). Moving next tostep 830, the processor 470 computes the value of the exponential term Eas in step 800, using the updated C_(L)(empirical). From the calculatedE, equation (3) may be expressed as in equation (4), which permitssolving for C_(L) algebraically. Hence, in block 835, the processor 470solves equation (4) algebraically for C_(L) to obtain an updatedC_(L)(approx). As in step 805, the algebraic solution for C_(L) in step835 is given by C_(L)(approx) in equation (5).

[0076] Next moving to step 840, the processor 470 determines ifC_(L)(empirical) is less than or equal to C_(L)(approx). Because step835 always results in a C_(L)(approx) that is between C_(L)(empirical)and the true Helmholtz capacitance, the conditionC_(L)(empirical)≦C_(L)(approx) indicates that the previous adjustment ofC_(L)(empirical) in step 825 resulted in a C_(L)(empirical) which wasless than or equal to the true Helmholtz capacitance. Accordingly,C_(L)(empirical) is guaranteed to be within Δ_(CL) below the trueHelmholtz capacitance, and C_(L)(approx) is guaranteed to be betweenC_(L)(empirical) and the true Helmholtz capacitance. The processor thusmoves to step 845, where the Helmholtz capacitance is estimated asC_(L)=C_(L)(approx). Alternatively, the Helmholtz capacitance may beestimated using the previous value of C_(L)(approx), which is guaranteedto be within Δ_(CL) above the true Helmholtz capacitance. IfC_(L)(empirical)≧C_(L)(approx) in step 840, however, then the processorrepeats back to step 825 to further adjust C_(L)(empirical).

[0077] Again examining step 820, if C_(L)(empirical)≦C_(L)(approx), thenC_(L)(empirical) is less than or equal to the true Helmholtzcapacitance, and the processor moves to step 850. From step 850, theprocessor 470 compares C_(L)(empirical) to C_(L)(approx). IfC_(L)(empirical)=C_(L)(approx), then both C_(L)(empirical) andC_(L)(approx) are equal to the true Helmholtz capacitance, and theprocessor 470 preferably estimates the Helmholtz capacitance asC_(L)(approx) in step 845. Alternatively, the processor 470 estimatesthe Helmholtz capacitance as C_(L)(empirical) in step 845. In addition,the Helmholtz capacitance may be estimated in step 845 as either thecurrent or previous value of C_(L)(empirical), since these values areguaranteed to be within Δ_(CL) of the true Helmholtz capacitance. IfC_(L)(empirical) is not equal to C_(L)(approx) in step 850, then theprocessor 470 moves to step 855. Steps 855 through 870 correspondapproximately to steps 825 through 840, except that C_(L)(empirical) isassumed to be less than the true Helmholtz capacitance in steps 855through 870 and is therefore adjusted in step 855 by adding Δ_(CL) toC_(L)(empirical).

[0078] Following step 855, the processor 470 moves to step 860 tocompute the value of the exponential term E as in step 800, using theupdated C_(L)(empirical). From the calculated E, equation (3) may beexpressed as in equation (4), which permits solving for C_(L)algebraically. Hence, in block 865, the processor 470 solves equation(4) algebraically for C_(L) to obtain an updated C_(L)(approx). As instep 805, the algebraic solution for C_(L) in step 865 is given byC_(L)(approx) in equation (5).

[0079] Next moving to step 870, the processor 470 determines ifC_(L)(empirical) is greater than or equal to C_(L)(approx). Because step865 always results in a C_(L)(approx) that is between C_(L)(empirical)and the true Helmholtz capacitance, the conditionC_(L)(empirical)≧C_(L)(approx) indicates that the previous adjustment ofC_(L)(empirical) in step 855 resulted in a C_(L)(empirical) which wasgreater than or equal to the true Helmholtz capacitance. Accordingly,C_(L)(empirical) is guaranteed to be within Δ_(CL) above the trueHelmholtz capacitance, and C_(L)(approx) is guaranteed to be betweenC_(L)(empirical) and the true Helmholtz capacitance. The processor thusmoves to step 845, where the Helmholtz capacitance is estimated asC_(L)=C_(L)(approx). Alternatively, the Helmholtz capacitance may beestimated using the previous value of C_(L)(approx), which is guaranteedto be within _(C L) below the true Helmholtz capacitance. In addition,the Helmholtz capacitance may be estimated in step 845 as either thecurrent or previous value of C_(L)(empirical), since these values areguaranteed to be within Δ_(CL) of the true Helmholtz capacitance. IfC_(L)(empirical)<C_(L)(approx) in step 870, however, then the processorrepeats back to step 855 to further adjust C_(L)(empirical).

[0080] When the Helmholtz capacitance C_(L) and load resistance R_(L)have been determined, a plurality of parameters of importance foranalyzing and optimizing a pacing system may be calculated, includingthe current delivered to the cardiac tissue at any instantaneous pointin time, the average current delivered to the cardiac tissue over theduration of the pulse, the total charge and the total energy deliveredto the cardiac tissue and to the leads, and the Helmholtz potentialafter pacing polarization. For instance, the current flowing through theheart tissue at time t, or i_(L)(t), is given by equation (6),neglecting R_(X): $\begin{matrix}{{i_{L}(t)} \approx {\frac{v_{i}}{R_{T} + R_{S\quad W} + R_{L}}^{\frac{{- {({\frac{1}{C_{T}} + \frac{1}{C_{B}} + \frac{1}{C_{L}}})}}t}{R_{T} + R_{S\quad W} + R_{L}}}}} & (6)\end{matrix}$

[0081] where e is the base of the natural logarithm.

[0082] Neglecting R_(X) as before, equation (7) represents the averagecurrent flowing through the heart tissue: $\begin{matrix}{{\overset{\_}{i}}_{L} \approx {\frac{v_{i}}{T_{PW}\left( {\frac{1}{C_{T}} + \frac{1}{C_{B}} + \frac{1}{C_{L}}} \right)}\left\lbrack {1 - ^{\frac{{- {({\frac{1}{C_{T}} + \frac{1}{C_{B}} + \frac{1}{C_{L}}})}}T_{PW}}{R_{T} + R_{SW} + R_{L}}}} \right\rbrack}} & (7)\end{matrix}$

[0083] where e is the base of the natural logarithm.

[0084] Again neglecting R_(X), equation (8) represents the charge Q_(D)delivered to the heart tissue from time t=0 to time t=T_(PW):$\begin{matrix}{Q_{D} \approx {\frac{v_{i}}{\frac{1}{C_{T}} + \frac{1}{C_{B}} + \frac{1}{C_{L}}}\left\lbrack {1 - ^{\frac{{- {({\frac{1}{C_{T}} + \frac{1}{C_{B}} + \frac{1}{C_{L}}})}}T_{PW}}{R_{T} + R_{SW} + R_{L}}}} \right\rbrack}} & (8)\end{matrix}$

[0085] where e is the base of the natural logarithm.

[0086] Finally, the energy J_(D) delivered to the heart tissue from timet=0 to time t=T_(PW), neglecting R_(X) as before, is given by equation(9): $\begin{matrix}{J_{D} \approx {\frac{v_{i}^{2}R_{L}}{2{\left( {R_{T} + R_{SW} + R_{L}} \right)\left\lbrack {\frac{1}{C_{T}} + \frac{1}{C_{B}} + \frac{1}{C_{L}}} \right\rbrack}}{\quad\left\lbrack {\left. \quad{{{\quad\quad}1} - ^{\frac{{- 2}{({\frac{1}{C_{T}} + \frac{1}{C_{B}} + \frac{1}{C_{L}}})}T_{PW}}{R_{T} + R_{SW} + R_{L}}}} \right\rbrack + \frac{Q_{D}^{2}}{2C_{L}}} \right.}}} & (9)\end{matrix}$

[0087] Thus, the present invention produces a very accurate impedancecharacterization of the lead/tissue interface, including both resistiveand reactive impedance components. Further, since buffers U4 and U5 havehigh-impedance inputs coupled directly to the pulse generator 468, thepresent invention is adapted to perform impedance measurements duringnormal pacing and defibrillating operation and with minimal interferenceto the pulse generator 468. In addition, and importantly, because theimpedance measurements occur during normal pacer operation, the paceroperation need not be suspended in order to collect impedance data.

[0088] Because the processor 470 controls the switches SW1, SW2, and SW3and also the sample signals, the processor 470 may be easily programmedto calculate lead/tissue impedance whenever desired. For instance, theprocessor 470 may calculate the lead/tissue impedance during everyn^(th) pacing pulse, where n can be an arbitrary integer. The periodicimpedance calculations can then be stored into main memory to beretrieved at a later date, perhaps by a physician who needs to verify oroptimize the implantable device 400. Storing the calculations in memoryalso allows the processor 470 to perform statistical analyses which areuseful for pacer maintenance, such as calculating minimum impedancemeasurements, maximum impedance measurements, and moving averages. Inaddition, if the implantable device 400 is capable of external controlthrough telemetry with a device external to the body, the processor 470can easily be programmed to calculate lead impedance duringmanually-induced test sequences. Hence, physicians have access to bothlong-term and immediate impedance data with which to optimize andmaintain the implanted device.

[0089] The alternative embodiments described above allow the processor470 to accurately calculate both the lead/tissue resistance R_(L) aswell as the Helmholtz capacitance C_(L) to any arbitrary degree ofaccuracy. Further, the alternative embodiments do not requiremeasurement of the shunt resistor voltage V_(CT)(T_(PW) ⁻) just prior tothe end of the pulse at time t=T_(PW) ⁻.

[0090] Numerous variations and modifications will become apparent tothose skilled in the art once the above disclosure is fully appreciated.It is intended that the following claims be interpreted to embrace allsuch variations and modifications.

We claim:
 1. An implantable apparatus for measuring the Helmholtzcapacitance of a patient's heart, comprising: an impedance circuit; anda processor coupled to said impedance circuit and receiving from saidimpedance circuit at least one signal from which the Helmholtzcapacitance is determined.
 2. The apparatus of claim 1 wherein: saidapparatus further comprises a pulse generator coupled to said impedancecircuit and including a resistor; said impedance circuit includes abuffer receiving a voltage associated with said resistor and asample-and-hold receiving an output signal from said buffer; and saidprocessor generates a control signal for operating said sample-and-hold.3. The apparatus of claim 2 wherein said at least one signal from whichthe Helmholtz capacitance is determined includes the voltage from saidsample-and-hold.
 4. The apparatus of claim 3 wherein said pulsegenerator generates a current pulse to be delivered to the patient'sheart during a pacing interval, said processor asserts said controlsignal within a predetermined time period before the end of said pacinginterval, and said sample-and-hold provides a voltage approximatelyequal to the voltage across said resistor at approximately the same timesaid processor asserts said control signal.
 5. The apparatus of claim 4wherein the Helmholtz capacitance is determined from an equation whichrelates the Helmholtz capacitance to the voltage provided by saidsample-and-hold.
 6. The apparatus of claim 1 wherein: said apparatusfurther comprises a pulse generator coupled to said impedance circuitand including a capacitor; said impedance circuit includes a bufferreceiving a voltage associated with said capacitor and a sample-and-holdreceiving an output signal from said buffer; and said processorgenerates a control signal for operating said sample-and-hold.
 7. Theapparatus of claim 6 wherein said at least one signal from which theHelmholtz capacitance is determined includes the voltage from saidsample-and-hold.
 8. The apparatus of claim 7 wherein said pulsegenerator generates a current pulse to be delivered to the patient'sheart during a pacing interval, said processor asserts said controlsignal within a predetermined time period after the end of said pacinginterval, and said sample-and-hold provides a voltage approximatelyequal to the voltage across said capacitor at approximately the sametime said processor asserts said control signal.
 9. The apparatus ofclaim 8 wherein the Helmholtz capacitance is determined from an equationwhich relates the Helmholtz capacitance to the voltage provided by saidsample-and-hold.
 10. The apparatus of claim 9 wherein an empiricalvoltage is determined using an empirical estimate C_(L)(empirical) ofthe Helmholtz capacitance.
 11. The apparatus of claim 10 whereinadditional empirical voltage values are determined based on additionalempirical estimates C_(L)(empirical).
 12. The apparatus of claim 11wherein the Helmholtz capacitance is determined to be the empiricalestimate C_(L)(empirical) which is associated with the empirical voltagethat is closest to the voltage provided by said sample-and-hold.
 13. Theapparatus of claim 9 wherein an approximate estimate of the Helmholtzcapacitance C_(L)(approx) is determined by substituting an empiricalestimate of the Helmholtz capacitance C_(L)(empirical) into one portionof said equation and solving said equation for the Helmholtzcapacitance.
 14. The apparatus of claim 13 wherein said C_(L)(empirical)is adjusted.
 15. The apparatus of claim 13 wherein if said C_(L)(approx)is within a predetermined range of said C_(L)(empirical), then theHelmholtz capacitance is estimated as said C_(L)(approx).
 16. Animplantable apparatus for measuring the resistance of a lead/tissueinterface, comprising: an impedance circuit; and a processor coupled tosaid impedance circuit and receiving from said impedance circuit atleast one signal from which said lead/tissue resistance is determined.17. The apparatus of claim 16 wherein: said apparatus further comprisesa pulse generator coupled to said impedance circuit and including ashunt resistor; said impedance circuit includes a buffer receiving avoltage associated with said shunt resistor and a sample-and-holdreceiving an output signal from said buffer; and said processorgenerates a control signal for operating said sample-and-hold.
 18. Theapparatus of claim 17 wherein said at least one signal from whichlead/tissue resistance is determined includes the voltage from saidsample-and-hold.
 19. The apparatus of claim 18 wherein said pulsegenerator generates a current pulse to be delivered to the patient'sheart during a pacing interval, said processor asserts said controlsignal within a predetermined time period after the start of said pacinginterval, and said sample-and-hold provides a voltage approximatelyequal to the voltage across said shunt resistor at approximately thesame time said processor asserts said control signal.
 20. The apparatusof claim 19 wherein said lead/tissue resistance is determined from anequation which relates the lead/tissue resistance to the voltageprovided by said sample-and-hold.
 21. A method for determining thepacing impedance of an implantable device, comprising: (a) determiningthe lead/tissue resistance; and (b) using said lead/tissue resistancedetermined in step (a) to determine the Helmholtz capacitance of apatient's heart.
 22. The method of claim 21 wherein step (a) comprises:(a1) applying a voltage V_(i) to a closed-loop current path comprising alead, heart tissue, and components of known impedance; (a2) measuring avoltage produced in said circuit by said voltage V_(i) shortly afterapplying said voltage V_(i) to said circuit; and (a3) calculating saidlead/tissue resistance based on the voltage measured in step (a2). 23.The method of claim 22 wherein step (a2) includes measuring a voltageacross a resistor.
 24. The method of claim 23 in which said lead/tissueresistance is calculated using a formula which includes said voltageacross a resistor.
 25. A method as in claim 21 wherein step (b)comprises: (b1) applying a voltage V_(i) for a fixed amount of time,T_(PW) seconds, to a closed-loop current path comprising a lead, hearttissue, and components of known impedance; (b2) measuring a voltageproduced in said closed-loop current path by said voltage V_(i) at atime t=T_(PW) ⁻, said time t=T_(PW) ⁻ occurring just before timet=T_(PW). (b3) calculating the Helmholtz capacitance based on thevoltage measured in step (b2).
 26. A method as in claim 25 wherein theHelmholtz capacitance is calculated using a formula which includes thevoltage measured in step (b2).
 27. A method as in claim 21 wherein step(b) comprises: (b1) discharging a tank capacitor for a duration ofT_(PW) seconds through a closed-loop current path comprising a lead,heart tissue, and components of known impedance; (b2) measuring avoltage across said tank capacitor at time t=T_(PW) ⁺, just after theend of said duration of T_(PW) seconds. (b3) estimating the Helmholtzcapacitance.
 28. A method as in claim 27 wherein step (b3) includescalculating an empirical voltage value across said tank capacitor basedon a predetermined empirical estimate of the Helmholtz capacitanceC_(L)(empirical).
 29. A method as in claim 28 wherein step (b3) furtherincludes calculating additional empirical voltage values across saidtank capacitor based on additional empirical estimates of the Helmholtzcapacitance C_(L)(empirical).
 30. A method as in claim 29 wherein step(b3) further includes estimating the Helmholtz capacitance to be theempirical estimate C_(L)(empirical) which is associated with theempirical voltage that is closest to said voltage across said tankcapacitor at time t=T_(PW) ⁺measured in step (b2).
 31. A method as inclaim 27 wherein step (b3) comprises estimating the Helmholtzcapacitance approximates said voltage across said tank capacitor usingan equation which includes the Helmholtz capacitance.
 32. A method as inclaim 31 including substituting said empirical estimate C_(L)(empirical)into a first portion of said equation and solving said equation for theHelmholtz capacitance to obtain an approximate estimate C_(L)(approx) ofthe Helmholtz capacitance.
 33. A method as in claim 32 includingcomparing said empirical estimate C_(L)(empirical) to said approximateestimate C_(L)(approx).
 34. A method as in claim 33 including adjustingsaid empirical estimate C_(L)(empirical).
 35. A method as in claim 33including estimating the Helmholtz capacitance to be said empiricalestimate C_(L)(empirical) if said empirical estimate C_(L)(empirical) iswithin a predetermined range of said approximate estimate C_(L)(approx).36. A method as in claim 33 including estimating the Helmholtzcapacitance to be said approximate estimate C_(L)(approx) if saidempirical estimate C_(L)(empirical) is within a predetermined range ofsaid approximate estimate C_(L)(approx).